In the postwar years, Cornplanter worked to learn more about European-American ways and invited Quakers to establish schools in Seneca territory. Disillusioned by his people's poor reaction to European-American society, he had the schools closed and followed his half-brother Handsome Lake's movement returning to the traditional Seneca way and religion. The United States government granted him about 1500 acres of former Seneca territory in Pennsylvania in 1796 for "him and his heirs forever", which became known as the Cornplanter Tract.

Still occupied by his descendants and holding his and many of their graves, the tract was planned by the federal government to be flooded as the site of a man-made reservoir after 1965 by completion of the Kinzua Dam on the Allegheny River. The remains of Cornplanter, his descendants, and an 1866 monument to him were relocated. Most of the remaining residents were forced to relocate to the Allegany Reservation of the federally recognized Seneca Nation of New York; they lost much of their fertile farmland.

The Dutch had settled in the area generations before, and Cornplanter's father, an Albany fur trader, was part of an established family. The Abeel family name was sometimes Gaelicized to O'Bail and O'Beal or anglicized to Abeele. John Abeel II (1722–1794) was connected to the Schuyler family, leaders in business and politics. The grandfather after whom he was named, Johannes Abeel I (1667-1711), was a trader and merchant who built up links with the indigenous people along his trade routes, and who served as the second mayor of Albany, later the capital of New York. The younger John Abeel was a gunsmith and was gladly accepted into the Indian community to repair their guns.

Cornplanter was raised by his mother among the Seneca. His Seneca name, Gaiänt'wakê (often spelled Gyantwachia), means "the planter," and another variation, Kaintwakon, means "by what one plants." As the Seneca and other Iroquois nations had a matrilineal system of kinship, Cornplanter was considered a member of his mother's clan, the Wolf Clan, which included many leaders in the relations between settlers and Indians, and gained his status from them.[4] Males of the Wolf clan had a traditional function as war chiefs.

Both sides initially told Indians that there was no need for their involvement. When the fighting between the Colonists and the British heated up, however; both sides tried to recruit the Iroquois as allies. The British offered large amounts of goods, specifically rum and other goods, and built on their long trading relationship. The Iroquois League met together at Oswego in July 1777, to vote on their decision. Although Guysutha and Cornplanter voted for neutrality, when the majority of chiefs voted to side with the British, they both honored the majority decision. Because of the status of the Seneca as war chiefs among the Iroquois, most of the Iroquois Confederacy followed suit. Still, bands often made their own decisions as the people were highly decentralized. The Iroquois named Sayenqueraghta and Cornplanter as war chiefs of the four nations that allied with the British: the Mohawk, Seneca, Onondaga and Cayuga.

Fighting on the frontier was fierce. Patriot forces under Colonel Thomas Hartley burned the Seneca village of Tioga. In reprisal, Cornplanter and Mohawk war chief Joseph Brant participated in the 1778 Loyalist-Iroquois attacks led by Captain Walter Butler and Butler's Rangers in Cherry Valley, New York. The Americans called these events the Cherry Valley Massacre. During this offensive many unarmed patriot civilians were killed or captured. During this campaign, Cornplanter's men happened to capture his father Johannes Abeel after burning his house. Cornplanter, who had once gone as a young man to see Abeel, recognized him and offered apology. He invited Abeel to return with the Seneca or to go back to his white family. When his father chose the latter, Cornplanter had Seneca warriors accompany him in safety.[citation needed]

After the victories of the Loyalist and Iroquois forces, commander-in-chief General George Washington commissioned Major General John Sullivan to invade Six Nation territory throughout New York and destroy Iroquois villages. At the Battle of Newtown, Sullivan defeated Iroquois and British troops. But Sullivan and his army of 5,000 men caused greater damage in their scorched earth campaign. They methodically destroyed Iroquois villages, farms, stored crops and animals between May and September 1779 throughout the Iroquois homeland (upstate and western New York).[6] Cornplanter, along with Brant, Old Smoke, and Lt. Colonel John Butler, fought a desperate delaying action in order to allow the escape of many refugees, both Native and non-Native, who went to Canada. Surviving Iroquois suffered terribly during the following months in what they called "the winter of the deep snow." Many froze or starved to death. Cornplanter and Seneca warriors continued to fight with the British against the Patriots, hoping to expel the colonists from their territory.

With Britain's final defeat in the war, Cornplanter recognized the need for a positive diplomatic relationship with the fledgling government of what the Iroquois called the "Thirteen Fires." He became a negotiator in disputes between the new "Americans" and the Seneca, as well as other indigenous tribes. He was a signatory of the Treaty of Fort Stanwix (1784), although this treaty was never ratified by the Iroquois. He also participated in later meetings with both presidents George Washington and Thomas Jefferson.[4]

After the American Revolution, Native Americans west of the Allegheny Mountains mounted a resistance to the European-American settlers in the Northwest Indian War in Ohio and Indiana, hoping to repulse the Americans. Cornplanter kept the Iroquois neutral in this conflict. In addition, he tried to negotiate with the Shawnee on behalf of the U.S.

In 1790, Cornplanter and his brother Half-Town (also a chief) traveled to Philadelphia to meet with President George Washington and Pennsylvania Governor Thomas Mifflin; they were protesting the current treatment of their people. Cornplanter and Half-Town extracted an agreement from Washington and Mifflin to protect Iroquois land.[7]

Cornplanter made many trips to the cities to strengthen relationships and talk with those who were interested in his people. He tried to learn the ways of the European Americans, as he saw it necessary for future relations between the Haudenosaunee and Americans. He was impressed by the beliefs and practice of the Quakers. He invited them to educate his son and develop schools in Seneca territory. He and his half-brother, the religious leader Handsome Lake, strongly opposed the use of liquor among the Seneca.

During the War of 1812, Cornplanter supported the American cause, convincing his people to do so as well. At one point he offered to bring two hundred warriors to assist the U.S., but his offer was refused.

He allowed Quakers into his village to help the Seneca learn new skills when they could no longer rely on hunting or the fur trade as a way of life. He also encouraged men to join the women working in the fields to help increase their farming economy. Quaker Run was developed as one of the first white settlements in Western New York; it is now abandoned and part of Allegany State Park.

Eventually, Cornplanter became disillusioned with his relationship with the Americans. To help fight the drunkenness and despair suffered by many Indians, his half-brother Handsome Lake preached that the Iroquois must return to the traditional way of life and take part in religious ceremonies. Cornplanter felt his people were poorly treated by the Americans.

He heeded Handsome Lake's prophecy that they should return to tradition and turn away from assimilation to white ways. He burned his military uniform, broke his sword, and destroyed his medals. He closed the schools but did not completely break relations with the Quakers; he retained a relationship of love and respect with them. Cornplanter also occasionally expressed his disdain for white men; upon taking a short ride on the first steamboat to navigate the upper Allegheny River, Cornplanter, while generally impressed with the boat, quipped that "white men will do anything to avoid using their muscles."[8]

In gratitude for his assistance to the state, the federal government gave Cornplanter a grant of 1,500 acres (6.1 km2) in Pennsylvania in 1796 along the western bank of the Allegheny River (about three miles (5 km) below the southern boundary of New York state), allotting it to him and his heirs "forever".[4] By 1798, 400 Seneca lived on the land, which was called the Cornplanter Tract or Cornplanter Grant (Cayuga: Gyonǫhsade:gęh[9]). In 1821 Warren County, Pennsylvania tried to force Cornplanter to pay taxes for his land, which he protested on the basis that the land had been "granted" to him by the U.S. government. After much talk, the state finally agreed that the Cornplanter Tract was exempt.

Cornplanter was a younger half-brother to Handsome Lake (Sganyadai:yo, ca. 1735-1815), a Seneca religious leader of the Iroquois. He was uncle to Governor Blacksnake (Thaonawyuthe, ca. 1760-1859),[10] a Seneca war chief. Like Cornplanter, Thaonawyuthe had an exceptionally long life for a man of his times.

Cornplanter married and had children. His son Henry Abeel (spelled Henry Abeele in federal documents) was an interpreter present at the Treaty of Canandaigua negotiations. In the winter of 1790, Cornplanter spent a year in Pennsylvania, during which he attended several Quaker gatherings. He was not converted by these gatherings, but he was impressed enough to send Henry and his other children to the Quaker school the following year. This sparked a continuing relationship between Cornplanter and the Quaker community.

Cornplanter's descendants typically used the last name Abeel (or variants thereof) during his lifetime. By the 20th century they had generally begun using the surname Cornplanter and continued to be prominent members of the Seneca community. Much of the Abeel/Cornplanter family died as a result of the 1918 flu pandemic.[11] Artist Jesse Cornplanter was his last known direct descendant (1889–1957).[12]

Cornplanter died on his Tract in 1836. He requested a grave with no marker. In 1866 the State of Pennsylvania installed a monument over his grave, "believed to be first monument erected in honor of a Native American in the United States."[13]

He was a dauntless warrior and wisest statesman of his nation, the patriarch of this tribe and the peacemaker of his race. He was a model man from nature’s mould. Truth, temperance, justice and humanity, never had a nobler incarnation or more earnest and consistent advocate then he. As we loved him personally, and revere the nobel, manly character he bore, we erect this tribute to his memory, that those who live after us may know and imitate his virtues.[13]

In 1965, the new federal Kinzua Dam at Warren, Pennsylvania was completed, soon permanently flooding all but a small corner of the Cornplanter Tract, as it created the Allegheny Reservoir for flood control.[14]
Cornplanter's grave, including the Cornplanter Monument, was subsequently moved to higher ground, at the Riverview-Corydon Cemetery, located in Elk Township, Warren County, Pennsylvania.[15][16] "The grounds are located west of the north central Pennsylvania town of Bradford just about 100 yards from the New York state line. The cemetery contains what are believed to be the remains of Cornplanter", and some 300 of his descendants and followers.[17] This property has eroded over the years, threatening the preservation of this important cemetery. The Seneca are reminded of their losses due to this damage.[18] In 2009 the state made plans to try to protect the cemetery.[19] The State of Pennsylvania erected an honorary marker at the site in 1966, after the original Cornplanter Tract was being submerged.[4] Most of the Seneca were relocated to lands in the Allegany Reservation in New York.

"The Cornplanter Medal was introduced in 1901 by Frederick Starr ... who had conducted research on the Iroquois Indians and wanted to give public acknowledgement to others who had contributed to the knowledge of the tribes. In order to help establish the medal, Starr and his colleagues raised money by selling a series of drawings of Indian life ... [by] Jesse Cornplanter."[22]

In 2003, the Warren, Pennsylvania-based non-profit organization Friends of Allegheny Wilderness proposed that a 3,022-acre inventoried roadless area, directly adjacent to the Cornplanter land grant within Allegheny National Forest lands, be designated as a wilderness area under the Wilderness Act of 1964, and that this wilderness area should be called the Cornplanter Wilderness in honor of Chief Cornplanter.[23]

The Cornplanter Stage inside the Key Bank Pavilion on the Warren County Fairgrounds in Pittsfield, PA, is named after him.

^Wallace, Paul A. W.; Hunter, William A. (1999). Indians in Pennsylvania. Harrisburg, Pa: Pennsylvania Historical and Museum Commission. p. 174. ISBN0-89271-017-9. 'For his many conciliatory acts, Pennsylvania gave him deeds for three tracts of land, only one of which he kept, the Cornplanter Grant, submerged in 1967 by the Kinzua Dam.'

^Wells, Randy. "Betts recounts life of lesser-known Native American leader". The Indiana Gazette Online. Indiana County, PA. Archived from the original on January 26, 2013. Retrieved September 23, 2012.Cite uses deprecated parameter |dead-url= (help) "Cornplanter is probably the most important of Native Americans, with the possible exception of Joseph Brant," Betts said. Betts is a retired Indiana University of Pennsylvania English professor and author."

The Speech of the Cornplanter, Half-Town, and the Great-Tree, Chiefs and Councillors of the Seneca Nation, to the Great Councillor of the Thirteen Fires. Signed at Philadelphia, the first day of December, 1790 – Library of Congress: pp. 140–142